The present invention relates to a data backup system for a computer, and more particularly to a data backup method for a hierarchical backup system that is formulated by combining a plurality of backup systems.
At present, various backup methods are used, including mirroring, snapshot, and backup on magnetic tapes, magnetic disks, and optical disks. All these methods have their advantages and disadvantages.
In mirroring, control is exercised to ensure that the same contents are always stored on a plurality of disks. Therefore, even if one of such disks becomes faulty, the current operation can be continued with another disk to minimize loss from a disk failure. However, if data deletion occurs due to a human operator error, software bug, or computer virus, the data on all the disks are deleted. In this sense, mirroring does not provide adequate data protection.
In situations where a snapshot copy of data is made on a periodic basis (this method is hereinafter referred to as the “snapshot method”), a previously saved snapshot copy remains unaffected even if the associated data is inadvertently deleted. If a failure occurs, however, recovery can only be achieved using the last snapshot copy. Therefore, this method is not suitable for operations where data is updated at intervals shorter than one minute. In general, it is difficult, in terms of cost, to manage many generations of snapshot copies. Old snapshot copies are sequentially overwritten by new ones so that the use of snapshot copies is not suitable for long-term storage of backup data.
The other means of backup include inexpensive disk drives, magnetic tapes, and optical disks. Making backups on inexpensive disk drives is generally superior in cost but inferior in performance to making snapshot copies. Making backups on magnetic tapes or optical disks is generally more superior in cost but more inferior in performance to making snapshot copies.
Although various backup methods are available, they have both advantages and disadvantages as described above. Under these circumstances, optimum methods have been employed for backup system construction in accordance with the properties of the data to be backed up. Also, a hierarchical backup system is now used in an increasing number of cases by hierarchically combining a plurality of backup means to exploit their advantages and offset their disadvantages (this system is hereinafter referred to as the “hierarchical backup system”). For example, the use of such a hierarchical backup system makes it possible to protect operations data by means of mirroring (first layer), make periodic snapshot copies to protect the data against software bugs and computer viruses (second layer), back up some snapshot copies on a magnetic disk to eliminate the risk of data loss, which may result when old snapshot copies are overwritten (third layer), and use a magnetic tape library to back up some backup data stored on a magnetic disk so that magnetic tapes can be removed and stored in a vault or at a remote place (fourth layer). It means that establishing a hierarchical backup system makes it possible to achieve failure recovery promptly and protect data not only from hardware faults but also from human operator errors, software bugs, computer viruses, and disasters.
However, if a failure occurs in a certain layer of a conventional hierarchical backup system, the failure affects the lower layers as well. If, for instance, a failure occurs in the second layer, no backup process can be performed in the third layer so that an improper backup results in the fourth layer.
The system disclosed by JP-A No. 36502/1996 consists of n units of equipment. In this system, k out of n units are activated for use while the remaining (n−k) units are designated as hot standby units. The system is configured so that the best k units are sequentially selected to determine the next active units. Even if a failure occurs in a certain unit within a hierarchical backup system that uses the above method, a certain standby unit becomes active and operates. Therefore, the hierarchical backup system can operate without causing any problem.
For the use of the above conventional technology, both active units and standby units need to be furnished. In a hierarchical backup system, which consists of various devices such a storage and server machine, various standby devices need to be furnished accordingly.